1. Field of the Invention
The present invention relates to an image processing apparatus and image processing program, and more particularly, to an image processing apparatus and image processing program that perform image data resolution conversion.
2. Description of the Related Art
Increasingly in recent years, image data obtained via reading of an original document by a scanner or the like, or image data obtained via photo-taking using a photo-taking apparatus such as a digital camera, has come to be recorded and stored on a recording medium such as a hard disk. Furthermore, such image data is also sent to other computers over a network, such as the Internet or the LAN.
Increasing the vividness of the image requires an increase in the resolution of the image data, which in turn increases the amount of data. Therefore, in order to effectively utilize the recording medium and network, means by which to reduce the amount of data is desired. As technologies by which to reduce the data amount of image data, various data compression technologies such as JPEG (Joint Photographic Experts Group) are available. However, even where a data compression technology is used, the degree to which the image data can be compressed and reduced in size is limited. Accordingly, even though it does result in decreased image quality, the amount of image data can be reduced significantly by reducing the resolution of the image data. However, if the resolution is decreased, the image quality is reduced as well. In other words, there is a trade-off between reducing the data amount and maintaining image quality. Therefore, the user is forced to prioritize between the image quality or the data amount.
Incidentally, a noticeable phenomenon in connection with the reduction in image quality caused by a reduction in the resolution is the appearance of gaps in fine lines. Japanese Laid-Open Patent Application H7-184043 describes a resolution conversion device that includes: an edge detection/fine line detection unit that determines (i) whether or not a target pixel of input multiple-tone image data represents an edge based on the density of the surrounding pixels in the image data, including the target pixel, and (ii) if the target pixel represents an edge, whether or not the edge is a fine line; resolution characteristic-maintaining resolution conversion means that converts input multiple-tone image data to a prescribed resolution while maintaining its resolution characteristic; tone characteristic-maintaining resolution conversion means that converts input multiple-tone image data to a prescribed resolution while maintaining its tone characteristic; and, output selection means that (i) operates in response to the output from the edge detection/fine line detection unit, (ii) where the detection unit determines that the target pixel represents the edge area of fine line, selects and outputs the output from the resolution characteristic-maintaining resolution conversion means, and (iii) where the detection unit determines that the target pixel represents something other than the edge are of a fine line, selects and outputs the output from the tone characteristic-maintaining resolution conversion means.
However, Japanese Laid-Open Patent Application H7-184043 entails the problem that a complex circuit construction is required in order to perform image data resolution conversion using the two different methods. Furthermore, where resolution characteristic-maintaining resolution conversion that reduces the resolution of all fine lines is performed, if fine lines are close to each other in the pre-conversion image data, the problem occurs that the fine lines become too close to each other and cannot be easily distinguished from each other in the post-conversion image data.